Register now to get rid of these ads!

Why aren't more frames built from aluminum?

Discussion in 'The Hokey Ass Message Board' started by wsdad, Apr 17, 2009.

  1. ProEnfo
    Joined: Sep 28, 2005
    Posts: 1,498

    ProEnfo
    Member
    from Motown

    Doug Nash / Bronco Buster

    http://www.draglist.com/artman/publish/daily_stories/article_001486.shtml

    CC
     
  2. Boyd built one,car and frame all ally;clever guy.
     
  3. Rex Schimmer
    Joined: Nov 17, 2006
    Posts: 743

    Rex Schimmer
    Member
    from Fulton, CA

    To make a steel rectangular tube and an aluminum rectangular tube l with the same bending stiffness you need to make the aluminum tube 1.4 times higher than the steel tube. Example, if you were using a 2 inch wide by 4 inch high X 1/8 wall stee tube and you were looking at bending in the 4 inch direction, you would need an aluminum tube that would be 2 X 5.6 x 1/8 wall. This is based upon making the "moment of inertia" of the aluminum tube in the long dimension to be 3 times that of the steel tube. The moment of inertia is a function of the heigth of the tube to the third power (cubed) so to be 3 time stiffer it needs to be the cube root of 3 (1.44) time higher.

    On thing to note here is that there is "stiffness" and there is "stress" If you load a 2x4x 1/8 wall tube, aluminum or steel, in bending they both have the same maximum stress as it is not a function of any of the material properties only the cross section of the tubes geometric properties, such as the moment of inertia.

    Not simple or obvious, but I would not hesitate to build an aluminum frame for a light weight car using 6061 T6551 tubing.The trick is to make sure that the stresses in the frame are very low which really extends the fatigue life of alulminum and explans why you can watch airplane wings move up and down and they don't fall off! (well most of them anyway!)

    All of this can be taken care of it you remember this advice: "When in doubt make it stout!"

    Rex
     
  4. pitman
    Joined: May 14, 2006
    Posts: 5,148

    pitman

    Rex's explanations help clear it up!
    Most structural equipment I've had a hand in, would behave when loaded, like a wet noodle in comparison to an appropriately sized steel counterpart.
    The stiffness was simply not there, esp when welding meant a softening on top of a lower Young's modulus.
    Now, carbon fiber would be another story.
     
  5. RichG
    Joined: Dec 8, 2008
    Posts: 3,919

    RichG
    Member

    Rex Schimmer just made my hero list...:D I saved his explanation to a word document, good stuff right there in layman terms, not geek-speak
     
  6. panic
    Joined: Jan 3, 2004
    Posts: 1,450

    panic

    Yup, he's right - it's the cube of the single plane dimension.
    Forgot Rule #1: engage brain before putting mouth in gear.
     
  7. There is no shortcut no knowledge, and the accumulated research of 250 years of continuum mechanics could never be summed up in entirety in a few paragraphs. There exists a immense body of knowledge in beam theory and material science, however, like most science and engineering knowledge, access to it seems limited to those who practice it. For the rest, its like searching in a huge dark library for a book who's name you don't know and would not recognize even it was in your hands. Obviously, this is a problem and likely one of the reasons the general public might be distrustful of scientists and engineers.

    There are number of factors that enter into the design of design of a particular part, a non-exhaustive list might include minimizing deflection, maximizing fatigue life, ultimate strength, cost of materials, cost of manufacturing and corrosion resistance. Each item on the list requires its own consideration, and although something like strength and deflection sound and look the same, they are not, and it is very possible to pass one and totally fail on the other.

    A crankshaft is something that needs to be designed for deflection and ultimate strength. Under the worst case loading conditions, we'd hope it wouldn't deflect more than a small amount (imagine a crank wound up like a rubber band). As it happens, the requirements for deflection require much more material than strength (compare the size of the crank shaft journals to the input shaft of the transmission, where deflection is less of a concern). Many things end up this way, where the stiffness design criteria displaces strength. I recall that cheap ass engine stand with the hemi on it, the motor nosed down at least 20 degrees, ultimate strength might have been okay, but deflection wasn't.

    The de Havilland Comet was the first commercial jet airliner. While it was an innovative design(and the most bad ass looking airliner ever), disregard for the effects of fatigue life and stress concentration led to explosive decompression of two of them. Deflection and ultimate strength were met, but someone dropped the ball on recognizing the effects of multiple compression cycles on the square window frames. We learned from the ordeal, and now all jets have round corners on the windows.

    Comparing 6061 aluminum to 1018 steel, Al has an elastic modulus of 70-80 GPa and a yield strength of 48MPa. 1018 steel has an elastic modulus of 190-210 GPa and a yield strength of 386MPa. This means that a steel part is 2.5 to 3 times stiffer than an aluminum part of the same geometry, and can take almost 10 times the load before undergoing plastic deformation. Fatigue life is more complex, and plays a big role in choosing aluminum parts, and as it was mentioned by other people, aluminum doesn't have a finite fatigue limit, unlike steel, which makes it an important factor in design, like the engineers at de Havilland learned.
     
    Last edited: Apr 18, 2009
  8. RichG
    Joined: Dec 8, 2008
    Posts: 3,919

    RichG
    Member

    Oh Lord, save me! The engineers have arrived!
    [​IMG]
     
  9. DocWatson
    Joined: Mar 24, 2006
    Posts: 10,273

    DocWatson
    ALLIANCE MEMBER

    While I agree fully with the Comet being the most elegant of all airliners I do have one point on your commentary.
    There was no instance of 'disregard' for the fatigue life around said windows. The simple fact was that the science involved was not yet understood. Yes there were pressurised examples before and some even had the dreaded square windows!
    It was a direct result of the two mid air catastrophic failures that we now know the stress and fatigue dangers involved.
    It is unfortunate that this lesson had to be learned with the death of so many people but isn't that how most insidious design failures in passenger aircraft are detected?
    One cant know what is going to happen if it has not happened before.
    Doc.
     
  10. Harry Bergeron
    Joined: Feb 10, 2009
    Posts: 345

    Harry Bergeron
    Member
    from SoCal

    BMW's aluminum car is the Z8, and they are now breaking in half at the cowl, from even minor curb and pothole encounters.
    Not cool for a $100K+ sports car.

    Audi's A8 is also aluminum, haven't heard any problems with them yet.
     
  11. DocWatson
    Joined: Mar 24, 2006
    Posts: 10,273

    DocWatson
    ALLIANCE MEMBER

    OH man I forgot about them! We had one come into the panel shop I was working in. If it had been any other car the repair bill would have been about 2g but you cant buy part to repair them from BMW.
    YOU HAVE TO REPLACE EVERYTHING FROM THE FIRE WALL FORWARD!

    It was almost 48 grand in parts alone, then once we had done the basic repair it had to go to BMW to be 'certified'.
    What a POS!
     
  12. Years ago i worked at a big truck salvage yard. We once weighed a 12 ft section of aluminum frame and a 12 ft piece of steel frame. The aluminum weighed a little more. Did the same with walking beams and front axles. the alumunum weighed a little more than the same item in steel. We came to the consenus that it takes thicker alumunum to do the same job as thinner steel . No weight savings. A guy was trying to come up with a super light semi. The lighter you can get your unladen weight the more payload you can haul. Oldwolf
     
  13. Darwin
    Joined: Oct 14, 2002
    Posts: 505

    Darwin
    Member

    Basic lesson here seems to be that aluminum needs to be significantly thicker in cross-section compared to steel to do the same job with some additional allowance for safety. In the case of a typical bucket tube frame even a properly engineered alloy version might be only 20 or 30 pounds lighter than a steel equivalent which would hardly be worth the significant hassle of using it--except purely for novelty purposes.

    Aircraft successfully use aluminum in monocoque structures that depend as much or more on their integrated design than exactly what material they are made from. This gives the structures multiple load paths which provide strength far in excess of what a simple tube material substitution would provide in something as simple as a bucket frame. I note that older fabric covered aircraft almost all use lightweight steel, not alloy, tubing for fuselage structure. Aluminum's weight reduction capabilities shine when used in elegantly engineered monocoque structures but tend to fall short when substituted in ordinary simple beam type structures even when allowances are made for inherent material strength.

    Aluminum also shines in componentry where thickness is less of an inhibiting factor such as engine blocks, manifolds, wheels, etc. as well as where its excellent heat transfer properties are an asset such as cylinder heads and pistons. A properly engineered SBC engine block can weigh over 100lbs less than does an iron version with little or no penalty in stiffness or performance. Alloy small-block in a bucket--good idea. Alloy frame on the same bucket--not so much.

    Naturally you'd never think to use aluminum, for instance, as an engine valve material, or a crankshaft, or pushrod, or any sort of spring. Conversely you might theoretically use steel as an aircraft wing skin but it'd have to be paper thin and would be very difficult to work with. In an aircraft aircraft wing structure even wood works better than steel except for use as attachment points. Each material to its own strengths.
     

Share This Page

Register now to get rid of these ads!

Archive

Copyright © 1995-2021 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. Terms of Service. Privacy Policy.

Atomic Industry
Forum software by XenForo™ ©2010-2014 XenForo Ltd.